1. Field of the Invention
The present invention relates generally to the field of optical power regulators for fiber optic networks. More specifically, the present invention discloses a broadband liquid-crystal optical power regulator.
2. Statement of the Problem
Optical power regulators are used in fiber optic networks to regulate the optical power levels. Optical power levels in an optical network can vary widely due to component malfunctions, network faults, or variations in the gain of optical amplifiers. Photoreceivers can be damaged if optical power levels become too great. When the optical power fluctuates, the optical power regulator adjusts its attenuation such that the output optical power level stays constant.
Optical amplifiers together with wavelength-division multiplexing (WDM) technology have become the standards for backbone fiber optic transmission networks. In WDM systems, multiple wavelength optical channels are launched into optical fibers. These optical signals are repeatedly amplified by erbium-doped fiber amplifiers (EDFA) along the network to compensate for transmission losses. The amplified signals reach the receiving end and are detected using WDM filters followed by photoreceivers.
One potential problem from the system point view is that there is gain competition from the EDFA. Gain competition can be understood by considering an EDFA having four input optical signals with almost equal power. If the EDFA has flat gain response, the output signals will have almost equal output power. However, if one of the input channels is degraded or fails, the output signals in the remaining three channels will experience a gain increase, thereby increasing their optical power. This can potentially overexpose the photodetectors and damage the receiver. This problem can be avoided by placing an optical regulator in the network to dynamically maintain optical power below a predetermined maximum level.
Conventionally, this problem is solved by using a variable optical attenuator. The optical attenuator is constructed by mounting a variable neutral density filter on a motor. By rotating the variable neutral density filter, variable optical transmission (attenuation) can be obtained. This type of attenuator, however, is based on opto-mechanical technology, which consumes a large amount of electrical power and has a limited lifetime. Opto-mechanical attenuators are especially weak when used in a dynamic power adjustment environment.
Variable attenuators based on liquid crystal technology are also commercially available. However, they are generally constructed so that in the absence of an applied voltage, their attenuation is at its maximum (i.e., lowest transmission). This is sometimes referred to as a normally "off" device. This design has two disadvantages. First, the applied voltage has to be "on" most of the time to let optical energy pass through the device. Second, if a black-out occurs, the attenuator will block the optical energy completely which is not preferred in most system designs.
U.S. Pat. No. 4,410,238 (Hanson) discloses a liquid-crystal attenuator that is polarization dependent and has a normally "on" design. However, the liquid crystal material used by Hanson has a steep slope for attenuation as a function of applied voltage, it is not well suited for use as a variable optical attenuator.
Other types of liquid-crystal attenuators, such as that taught by U.S. Pat. No. 5,015,057 (Rumbaugh et al.), use a polymer-dispersed liquid crystal (PDLC) film to scatter optical energy. However, residual insertion loss is high and a high voltage must be applied to achieve high transmission with this approach. PDLC attenuators are also always "off" devices, which again is not preferred from the system perspective, as previously discussed. Finally, all liquid crystal attenuators suffer slow switching times due to the small optical birefringence (.DELTA.n of about 0.17) or high viscosity of conventional liquid crystal materials.
3. Solution to the Problem
The present invention is a broadband optical power regulator that utilizes the unique broadband nature of birefringent crystals and a polarization modulator to achieve a constant optical power output. The liquid-crystal variable attenuator (LCVA) used in the present power regulator also has features that distinguish it from the prior art, such as Hanson. A liquid crystal material is employed having a large optical birefringence but a small dielectric anisotropy. The large optical birefringence results in a thin liquid crystal cell, that dramatically reduces the switching time of the attenuator. The small dielectric anisotropy (the difference between parallel and perpendicular dielectric constants, .DELTA..epsilon.=.epsilon..sub.// -.epsilon..sub..perp.) results in a shallow slope for the attenuation curve as a function of the control voltage. This produces a more stable attenuation level for the regulator and a much greater number of controllable gray levels. A three-level driving scheme can be implemented to achieve high-speed switching. This feature is especially desirable because if an optical power surge occurs on the network, the regulator should respond to this noise as quickly as possible to prevent damage to the optical receiver. The present design is also relatively insensitive to changes in the applied voltage or temperature.